In the current practice of manufacturing gypsum products, used as the basic raw material is hemihydrate gypsum CaSO.sub.4.0.5H.sub.2 O of construction grade, produced at specialized plants. To obtain this type of gypsum, natural gypsum stone (CaSO.sub.4.2H.sub.2 O) is crushed and ground to powder in ball mills. Then in special boilers, the crushed and ground gypsum stone is heat-treated with mixing at a temperature of up to 180.degree. C. for 4 to 8 hours. After cooling, the gypsum is packed and, protected against moisture, sent to a gypsum product manufacturing facility.
When hemihydrate gypsum CaSO.sub.4.0.5H.sub.2 O is used for manufacturing construction products, it is subjected to watering a few minutes before filling molds (the weight water to gypsum ratio being 0.4-0.6). It takes 5 to 15 minutes for gypsum to cure in a mold. After separation from the mold, the product is dried at a temperature of 50.degree. to 60.degree. C. for 8 to 16 hours, depending on the product's size.
All products so manufactured feature high porosity and hygroscopicity and, therefore, can be used only for interior work (partitions, floor tiles, decorative and sound absorbing panels). Moreover, the known process is complicated for it requires use of a starting material specially prepared in advance.
Also known is a process of manufacturing gypsum products, comprising crushing and grinding of gypsum stone-natural dihydrous gypsum CaSO.sub.4.2H.sub.2 O--to grains with a specific surface of 2,500 to 2,800 cm.sup.2 /g, with subsequent mixing of the ground gypsum stone with water in amounts at which the weight water to gypsum ratio is 0.1-0.2. Then, the resulting mixture is poured into a mold, and products are formed. To compact the mixture, vibration is used. The molded products are subjected to hydrothermal treatment in water steam at a pressure of 7 to 7.5 atm. This hydrothermal treatment is carried out in an autoclave where dihydrous gypsum is dehydrated to a hemihydrate. For subsequent strengthening of the products by hydration they are immersed in water till saturation is reached (cf. USSR Inventor's Certificate No. 502,853).
This process is disadvantageous in that the mixture being molded is highly humid, which, in combination with the subsequent hydrothermal treatment, results in products featuring a porous structure with a water absorption of 18 to 20% and a low frost resistance of 10 to 15 cycles (by one cycle is meant freezing to a temperature of -15 to -20.degree. C. for 4 hours, followed by defreezing in water for 4 hours). In addition, resorting to heat treatment in an autoclave which is a batch-type piece of equipment does not permit manufacturing gypsum products in an assembly line process, nor does it allow the process as a whole to be automated.
There is known still another process of manufacturing gypsum products, disclosed in USSR Inventor's Certificate No. 62,785. According to this process, crushed and ground natural dihydrous gypsum CaSO.sub.4.2H.sub.2 O with a possible presence of fillers is humidified to 6-10% if the products are to be made by pressing or compaction; in the case where the products are made by molding or vibration, the batch is humidified by adding a large quantity of water to attain a plastic consistency. The products molded in one fashion or another are then subjected to hydrothermal treatment, for which purpose they are placed in an autoclave with water steam being supplied thereinto at a pressure of at least 1 atm. The treatment under pressure continues for 3 to 6 hours. According to the above-cited Inventor's Certificate, as dihydrous gypsum is transforming to a hemihydrate, even at low pressures (1 to 1.5 atm) it recrystallizes with cogesive forces appearing between the hemihydrate crystals, which renders products much stronger directly in the course of the hydrothermal treatment. Then, the water steam is slowly released. After the hydrothermal treatment and cooling, the hemihydrate gypsum in the products is hydrated due to the water remaining in pores, whereby the products are additionally strengthened.
This process suffers from the same disadvantages inherent in the one described previously. It should be noted that products manufactured according to the above processes, by molding batches of a plastic consistency (the weight water to gypsum ratio being above 0.1), simply cannot feature improved frost and water resistance. When products molded by pressing are hydrothermally treated in an autoclave with steam, it is impossible to attain to porosity of less than 14-15% and the required frost resistance (35 cycles and above).